The present disclosure relates generally to a switched mode power supply and more especially to a switched mode power supply with a multifunction pin.
A switched mode power supply commonly utilizes a power switch to control the current flowing through an inductive device. In comparison with other kinds of power supply, switched mode power supplies usually enjoy compact size and excellent conversion efficiency, and are accordingly popular in the art.
FIG. 1 demonstrates a conventional switched mode power supply 10 with a flyback topology. Bridge rectifier 20 performs full-wave rectification, converting the alternative-current (AC) power source from an AC mains outlet into a direct-current (DC) input source, and providing both an input voltage VIN at an input power line IN and a ground voltage at a ground line GND. The input voltage VIN could have an M-shaped waveform or be substantially a constant. Controller 26 is an integrated circuit with pins VCC, QRD, COMP, BNO/BOI, and GATE, connected to peripheral devices. Via pin GATE, power controller 26 provides a PWM signal VGATE, periodically turning ON and OFF a power switch 34. When the power switch 34 is ON, a primary winding PRM of the transformer energizes. When it is OFF, the transformer de-energizes via a secondary winding SEC and an auxiliary winding AUX to build up an output voltage VOUT for load 24 and an operation voltage VCC for power controller 26.
A voltage divider consisting of resisters 28 and 30 detects voltage drop VAUX across the auxiliary winding AUX, to provide a quasi-resonance signal VQRD to pin QRD of controller 26, which could accordingly perform valley switching to reduce the switch loss in the power switch 34.
The regulation to output voltage VOUT could be achieved by accumulating the error between output voltage VOUT and an expected target voltage to modulate the duty cycle of the PWM signal VGATE. Resistors 44 and 46 divide output voltage VOUT to provide a divided result, which LT431 uses to compare with an internal reference voltage, 2.5V for example, and to generate an error signal. Photo coupler 42 optically transmits, from a secondary side to a primary side, the error signal, which is accumulated over time at pin COMP on a compensation capacitor 50. A resistor 52 connected to operation power line VDD inside controller 26 provides a driving force to push up the compensation voltage VCOMP while photo coupler 42 provides an opposite driving force to pull down the compensation voltage VCOMP. PWM generator 40 generates PWM signal VGATE in response to the compensation voltage VCOMP.
Resistors 53 and 54 are connected between the input power line IN and the ground line GND, for brownout and brownin detection. As known in the art, a brownout is an intentional or unintentional drop in voltage in an electrical power supply system, and this voltage reduction may be an effect of disruption of an electrical grid for example. A brownout could cause a switched mode power supply to malfunction if there is no corresponding protection. Controller 26 detects the input voltage VIN via pin BNO/BNI. If a brownout is found, controller 26 constantly turns off power switch 34, to shut down and protect the switched mode power supply 10. Here in this specification, a brownin refers to an increment in the input voltage VIN that controller 26 could resume to turn on and off power switch 34 periodically. The switched mode power supply 10 might shut down if the input voltage VIN is under a brownout voltage, 60V for example, and resume switching the power switch 34 if the input voltage VIN recovers to exceed a brownin voltage, 70V for example.
Even though the switched mode power supply 10 could accurately perform brownin and brownout detection, an extra pin BNO/BNI is necessary. Pin count is crucial in the art, as it largely determines the total cost of an integrated circuit. Accordingly, it is preferable to have a less pin count and keep the same functionalities as well at the same time for an integrated circuit.